The present invention relates generally to the treatment of contaminated media, including solids, sludges, soils, suspensions, sediments, and liquids containing high concentrations of heavy metals by the incorporation or mixing of slags resulting from lead smelting processes into the contaminated media thereby lowering the level of total or leachable heavy metals from the contaminated media.
In the United States, solid wastes are classified as “non-hazardous” or “hazardous” by the United States Environmental Protection Agency (USEPA) pursuant to 40 CFR Part 261. Any solid waste can be defined as hazardous either because it is “listed” in 40 CFR Part 261 (Subpart D), or because it exhibits one or more of the characteristics of a hazardous waste as defined at 40 CFR Part 261 (Subpart C). These characteristics are: (1) ignitability, (2) corrosivity, (3) reactivity, and (4) toxicity. 40 CFR Part 261.24(a) contains a list of contaminants and their associated maximum allowable concentrations. If a particular contaminant in a contaminated media exceeds its maximum allowable concentration when subjected to a “Toxicity Characteristics Leaching Procedure” (TCLP) as specified at 40 CFR Part 261 Appendix 2, then contaminated solid waste is classified as “hazardous” and must be handled, transported, treated, and disposed as a hazardous solid waste.
During the TCLP test, contaminated media is mixed with either a dilute acetic acid in de-ionized water (TCLP fluid 2) or, depending of the pH of the contaminated material, in de-ionized water containing a sodium hydroxide buffer (TCLP fluid 1) to determine the concentration of leachable contaminants from a contaminated media if they were to be deposited in a landfill. The test was developed to simulate the potential reaction between the existing (acidic) environment in a landfill created by the combination of rainwater and the decomposition of organic matter existing in the landfill with the contaminated media.
The USEPA established the Land Disposal Restrictions (LDR) program to ensure hazardous waste are properly treated to destroy, stabilize, or immobilize hazardous chemical components before land disposal so as to not pose a threat to human health and the environment.
The LDR requires that hazardous solid waste are treated such that heavy metals do not leach from the solid waste at levels above the maximum allowable concentration prior to placement in a surface impoundment, waste pile, landfill or other land disposal unit as defined in 40 CFR.260.10.
As noted in “A critical review on secondary lead recycling technology and its prospect”, W. Zhang et al., Renewable and Sustainable Energy Reviews 61 (2016), pages 108-122, lead is a versatile and strategically important industrial metal resource, and its production, recycling, application and consumption must be conducted such that any resulting lead emissions resulting from primary and secondary lead smelting activities are protective of the public health and environment.
There are two basic types of lead resources—primary lead resources such as lead ores in the form of minerals such as galena (PbS), cerussite (PbCO3) and sulfuric acid galena (PbSO4), and secondary lead resources mainly produced through the recycling of discarded lead-acid batteries. Even with the development of more efficient smelting furnaces and pretreatment equipment, the basic flow of lead-acid battery recycling operations has not significantly changed over the years.
In general, spent leads-acid batteries are first crushed in a hammer mill where the lead metal, polypropylene, lead grids and other solids are effectively separated from the spent lead paste. The resulting spent lead paste is composed of lead sulfate (≈60% by weight), lead dioxide (≈28%), lead oxide (≈9%), and a small amount of metallic lead.
A common method of further refining spent lead paste is by an alkali-fusion process as described by Lassin, et al. in “Estimated thermodynamic properties of NaFeS2 and erdite (NaFeS2:2H2O), Applied Geochemistry 2014 (45), pages 14-24. In general, lead is chemically reduced to its metallic form (between 327° C. (lead melting point) and 650° C. (lead boiling point) by eliminating the sulfates as sulfides and sulfidizing the chalcophile metals (e.g. Cu, Ag). The sodium alkali-fusion method uses alkalizing agents (e.g. Na2CO3, NaOH), desulfurizing agents (e.g. iron), and reducing agents (e.g. coke) to produce a hard (antimonial) lead which still may contain impurities (e.g. Cu, Sb, As, Sn, etc.). The hard lead is then turned into soft lead during a second stage where the traces of chalcophile metals are sulfidized, the Sn, As and Sb are oxidized, and the Ag and Bi in the Ag—Zn—Pb and Ca—Mg—Ag alloys that float on the molten lead are solubilized.
The sulfidic slag produced by the sodium alkali-fusion process however is primarily comprised of a particularly unstable sodium-iron-sulfide slag (sometimes referred to as sodium-iron-sulfide slag or sodium-iron-sulfide scoria), that if exposed to air, rapidly decomposes into a blackish powder. This sodium-iron-sulfide slag is primarily comprised of NaFeS2, and due to the rapid topochemical reaction occurring in the presence of minimal amounts of water (atmospheric or otherwise), may reversibly or irreversibly convert to its hydrated mineral form—erdite (NaFeS2:2H2O).
A similar sodium-iron-sulfide material is also described in a paper entitled “Zur Kenntnis des Natriumthioferrates(III), Monatshefte fur Chemie 114, (1983), pages 145-154 authored by Herbert Boller Herbert Blaha. These authors describe a mixed-valence compound Na3Fe2S4, which is oxidized and hydrated in air to NaFeS2:xH2O, where x≈2. It was further shown by thermogravimetric analysis (TGA) that this hydrate loses the water reversibly between 80° C. and 140° C. The formation of NaFeS2:xH2O, where x≈2 and NaFeS2 were described as “topotactic”, meaning there was a structural change of the crystalline solid by the addition (or loss) of water such that the final lattices of each are related by one or more crystallographically equivalent, orientational relationships.
A source of Na3Fe2S4, NaFeS2, or their oxidized and hydrated form (NaFeS2:xH2O), where x≈2, are contained within the aforementioned sulfidic slag byproduct generated from secondary lead smelting operations.
Two common methods for treatment, stabilization, precipitation, or otherwise removal of dissolved heavy metal ions from contaminated media are hydroxide and sulfide precipitation.
In hydroxide precipitation, alkaline chemicals (e.g. lime or caustic) are used to adjust the pH of the contaminated media to a pH range where targeted heavy metal ions are least soluble and will precipitate as metal-hydroxides. Depending upon the presence of competing chemical species in the contaminated media, certain heavy metal-hydroxides (e.g. hydroxides of zinc, nickel, copper, lead, cadmium) are amphoteric compounds and exhibit minimum solubility in the pH range of 8 to 12. FIG. 1 demonstrates this characterization.
Two patents that illustrate the concept of treating a contaminated media by hydroxide precipitation are disclosed in U.S. Pat. No. 4,671,882 to Douglas, et al. and U.S. Pat. No. 5,916,123 to Pal, et al. Both disclose multi-step treatment methods to chemically convert metal-bearing solid and liquid waste materials to a non-leachable form by adjusting the pH of the contaminated media to form metal-hydroxides.
Again referring to FIG. 1, since sulfide ions have a greater affinity for the heavy metal ion than the hydroxide ion, sulfide precipitation of metal ions from solutions result in much less soluble metal-sulfide precipitates when compared to their metal-hydroxide equivalents.
Many patents disclose methods for removing heavy metal pollutant ions from solutions based on formation of metal-sulfide precipitates, including U.S. Pat. No. 3,740,331 to Anderson et al., whereby a sulfide ion and a metal ion that forms a metal-sulfide having a higher equilibrium sulfide ion concentration than the heavy metal pollutant to be removed are added to a liquid solution to be treated.
Further, since many of the heavy metal ion pollutants (e.g. zinc, nickel, tin, cobalt, lead, cadmium, silver, bismuth, copper, mercury) are less soluble than iron or manganese sulfides, various methods of producing iron or manganese sulfides directly in solutions containing heavy metal ion pollutants have been disclosed (U.S. Pat. No. 4,102,784 to Schlauch, U.S. Pat. No. 6,153,108 to Klock, et al.), while U.S. Pat. No. 4,422,943 to Fender, et al. discloses various methods to admix an aqueous slurry of FeS2 with a heavy metal bearing solution at a pH>7.
Other patents, for example U.S. Pat. No. 6,991,593 to Price, et. al disclose a two-step method to treat metal-bearing contaminated solid media by first adjusting the pH to a range from about 8.5 to 12.5 and then adding sufficient sulfide containing reducing reagent to the metal-bearing solid waste to reach an oxidation reduction potential less than about 50 mV in an extract from the metal-bearing waste.
These aforementioned patents, in particular those related to treatment of contaminated media containing a mixture of heavy metals, are based upon the concept that the mixture of heavy metals in the contaminated media will preferentially precipitate from the solution as metal-sulfides (or metal-hydroxides) based on the their respective solubility product (Ksp). A comparison of the solubility of various metal-sulfides are presented in Table 1 below:
TABLE 1Solubility of various metal-sulfides at 25° C.(*)FormulaMineral Namelog(Ksp)MnSAlabandite−0.003NaFeS2−1.228FeSFeS(am)−2.990FeSMackinawite−3.540FeSPyrrhotite−3.679FeSTroilite−3.874NaFeS2:2H2OErdite−5.500AsSRealgar−7.800Pd4S(s)−8.837ZnSWurtzite−9.189NiSNiS(alpha)−9.577ZnSSphalerite−11.488CdSGreenockite−14.820PbSGalena−14.836Cr2S3(s)−16.704CuSCovellite−23.731HgSMetacinnabar−26.850CoS2Cattierite−27.183SnS2Berndtite−32.151CuFeS2Chalcopyrite(alpha)−33.669Cu2SChalcocite(alpha)−34.755Ag2SAcanthite(alpha)−36.070HgSCinnabar(alpha)−39.006PdSVysotskite−44.806PtSCooperite−60.932FeSb2S4Berthierite−61.059As2S3Orpiment−65.110FeAsSArsenopyrite−92.129(*)From the Bureau de Recherches Geologiques et Minieres (http://thermoddem.brgm.fr/) last accessed Aug. 8, 2016
Given the high volumes of sodium-iron-sulfide slag produced annually during the refining and smelting of spent lead-acid batteries, developing methods to beneficially and economically reuse these slags are of interest.
The use of sodium-iron-sulfide slag as a hazardous treatment reagent, either by itself, or in combination with other treatment reagents or protocols, provides a novel methodology for managing and treating contaminated media containing hazardous concentrations of a single or multiple heavy mental contaminant(s),